The center of this bustling city is the "Haram" devoted to one of Islam's Imams. Many argue that Mashhad is extremely religious in nature due to this, which is an argument that has merit, but the commercial growth and urban development in this city brings a certain secularism to Mashhad as well.
If you plan to visit this city, especially as you get closer to the Haram, be prepared to dress and comport yourself in a manner that is respectful of Islam. People will stop you in the street to comment on your clothing, and chastise you. The Haram itself is gorgeous, the architecture and design is stunning, and it is bustling with devotees.
Other than that, there are tons of places to go for commercial endeavors. Mashhad has a huge bazaar as well as tons of other shops and factories/warehouses. These, and the Haram, are the main attractions to this city.